She says about thousands of other prosecutions against welfare recipients could be affected.

"The implication of this High Court decision today is that the DPP cannot rely on backdated legislation to create an obligation on people to tell Centrelink of something that didn't exist at the time," she told The World Today.

"The court has referred to that as a statutory fiction.

"So what this means is that 15,000 prosecutions that were progressed during that period when that legislation was said to apply, are now in doubt."

Ms Hilton says in many cases, those facing prosecution did not set out to defraud Centrelink.

"They haven't tried to rort Centrelink payments," she said.

"These are people in most circumstances who have received an overpayment and in most cases have paid that money back but are being prosecuted because they haven't understood an obligation and they've failed to communicated to Centrelink that they've had a change in their circumstances."